This invention relates to the construction of high-density circuits, for use in various applications.
The circuits of the present invention are very stable, i.e. they are substantially unaffected by changes in temperature and humidity, and do not warp or change in size in any great degree. They are also highly resistant to corrosion. The circuits of the invention are also extremely rugged, resistant to abrasion, and resistant to long-term exposure to adverse environmental effects, such as extremes of temperature or humidity, vibration, salt, and other effects.
The conductors of the circuits in the present invention are extremely narrow and tightly-packed, yet they can carry relatively high voltages and/or currents. The conductors, and the spaces between them, are less than about 0.005 inches wide. The invention can therefore be used in the fabrication of ultra-high precision large linear circuit arrays for thermal or electrostatic print heads for generating high-resolution images. The invention can also be used to form print heads which produce black-and-white or color halftones.
Various methods have been proposed, in the prior art, for manufacturing high-density circuit. One early example is given in U.S. Pat. No. 2,932,710. The latter patent shows a technique for forming a plurality of metallic electrical conductors in an automobile window, for the purpose of heating (and defrosting) the window. The patent shows conductors having a spacing which ranges from about 50 to about 200 lines per inch, and having line widths of from 0.0001 to 0.002 inches. The conductive metal is silver, which is applied by coating the glass substrate with silver, or by an evaporation or vapor deposition technique.
In U.S. Pat. No. 4,604,298, a conductive material is squeezed through a fine screen, such as a molybdenum sheet which may be about 0.001 inches thick, and onto a substrate. The patent shows a method of producing conductors as narrow as 0.00175 inches, with a spacing of less than 0.005 inches. However, the patent states that the lengths of the circuit lines must be less than about 0.050 inches, in order to preserve the integrity of the molybdenum screen. The technique of the cited patent is difficult to control.
U.S. Pat. No. 4,574,094 discloses a method of placing a set of electric conductors on a ceramic material using a metalization technique. That is, a layer of conductive material, arranged in a suitable circuit pattern, is applied directly to a chemically-treated dielectric surface and fired, so that the conductive material binds to the dielectric. Although the width of the lines of the conductors can be as low as about 25 microns (less than 0.001 inches), the metallizing process has the disadvantage that it is difficult to form conductors having significant thickness (height). Metalization typically results in thicknesses of 2000-3000 Angstroms (about 0.000008 to about 0.000012 inches). With plating-up techniques, this thickness can sometimes be increased. However, with increased thickness, surface adhesion becomes a problem, due to stresses that build up during the metalization process, and these stresses are not easily overcome. These shortcomings limit the current-carrying capacity of the conductors. Since the circuit is on the surface, damage to the circuit lines is also a common problem, and extreme care must be exercised during fabrication and assembly.
U.S. Pat. Nos. 4,508,753 and 4,508,754 disclose methods of making fine-line circuits on an insulating substrate. The patents disclose circuits which are engraved in the substrate, but do not disclose how dense and how large these circuits may be.
U.S. Pat. No. 4,454,167 discloses another process for producing a high-density circuit on an insulating substrate. The method involves the use of photosensitive materials and metallic slurries to create fine line patterns. The circuit pattern is applied directly to the surface of the substrate.
Another method of applying a conductive material to a ceramic substrate involves the screening of the material onto the entire substrate surface, firing the substrate, and then selectively removing the unwanted material by photolithography, plasma or other etching techniques. In the case of photolithographic techniques, the tendency for the circuit path to separate from the substrate during processing is quite high. One cause of line breaks is from microscopic surface imperfections in the dielectric or conductive layers, and non-adhesion sites between the dielectric and conductive layers that allow the etchant to penetrate between the conductive layer and substrate dielectric by running under the fine-line circuit paths, from etched areas, and breaking the circuit from below. This problem becomes more critical as line widths decrease and the number of circuit lines increases. The state of the art is not sufficiently advanced in both design and economics to allow the use of plasma etching techniques to construct the large scale circuit arrays described in this invention.
U.S. Pat. No. 4,289,364 discloses a method of connecting a flexible circuit to a circuit which is etched in a glass substrate. The grooves in the glass are comparatively wide, of the order of about 0.032 inches.
Various means have also been used to generate images on paper and other media. A print head for a thermal printer includes resistive materials placed between circuit paths. The resistive materials are heated and brought in contact with the wax-coated printing paper or ribbon. U.S. Pat. No. 4,604,298 discusses the application of fine-line circuit construction to the fabrication of single-layer thermal print heads.
An electrostatic high-speed printer generates charged sites on a drum or belt, either by an optical scanning technique or by using a linear circuit array disposed near the drum or belt. It has been known to construct such linear circuit arrays using conventional printed circuit technology. These arrays suffer from a very high reject rate and non-linearity of the pixel array due to the imprecise methodology and materials of construction. They also do not have the operating temperature range, mechanical stability, or ruggedness of this invention.
All of the printer or image-generation devices described above seek to produce a very uniform and precise pixel format with as many pixels per inch as possible. The human eye is a very good image integrator and can detect extremely small differences in a closely spaced pixel array or closely spaced series of lines. The eye can detect these differences, even though individual pixels or lines cannot be resolved. This is the basic reason that the so-called halftone technology has been so successful for the printing of photographic images with inks.
In the case of thermal printers, a ceramic substrate with a dielectric coating, and a fired-on conductive pattern, comprises the linear array. As mentioned above, U.S. Pat. No. 4,604,298 discusses the application of high-density circuit construction to the manufacture of thermal print heads. But a thermal print head made according to the method of the latter patent has the disadvantage that its conductors are limited in length to about 0.050 inches. Also, the length of the circuit is subject to variation due to the stretching of the screen. Adhesion to the substrate, and uniformity of circuit thickness and width, are highly dependent on the viscosity of the conductive material, the thickness of the mask, and operator technique.
Printing heads are also fabricated from copper-clad circuit board materials, in which case the one makes the circuit board with the print head array lines in the center of the board, with a pixel density one-half that of the finished print head. The board is then fabricated using conventional photolithographic and printed circuit etching techniques. It may also be plated up to increase the circuit thickness or aspect ratio. The board is then sliced in half along its center, and the two halves are placed either face-to-face, with a thin dielectric separator to prevent shorts, or back-to-back. By offsetting the boards, the desired array density is achieved. Arrays having conductor widths of 0.003 inches, with densities of up to 240 lines per inch, have been produced by this general method. These arrays suffer from a very high reject rate and non-linearity of the print head array, due to the imprecise methodology and materials of construction. These circuits also do not have the operating temperature range, mechanical stability, or ruggedness of the present invention.